The eyes do not stay perfectly still during attempted fixation; fixational eye movements and saccadic intrusions (SIs) continuously change the position of gaze. The most common type of SI, square-wave jerk (SWJ), consists of pairs of horizontal saccades: the first saccade moves the eye away from the fixation target and, after a short interval, the second saccade brings it back towards the target. SWJs are prevalent in certain neurological disorders, including progressive supranuclear palsy (PSP). Here we developed an objective method to identify SWJs. We confirmed that SWJs are more frequent, larger and more markedly horizontal in PSP patients than in healthy human subjects. We also discovered that the loss of a vertical component in fixational saccades and SWJs is the eye movement feature that best distinguishes PSP patients from controls. We moreover found that in PSP patients and controls, the larger the saccade the more likely it is part of a SWJ. Further, all saccades produced by PSP patients had equivalent properties whether they were part of a SWJ or not, suggesting that normal fixational saccades (microsaccades) are rare in PSP. We propose that fixational saccades and SIs are generated by the same neural circuit, and that, both in PSP patients and in healthy subjects, SWJs result from a coupling mechanism that generates a second corrective saccade shortly after a large fixation saccade. Due to brainstem and/or cerebellum impairment, fixational saccades in PSP are abnormally large, and thus more likely to trigger a second corrective saccade, giving rise to SWJs.

Barrow Neurological Foundation (to SLM and SMC), Arizona Biomedical Research Commission (to SMC), National Science Foundation (awards 0643306 and 0852636 to SMC), NIH (grant EY06717 to RJL), the Office of Research and Development, Medical Research Service, Department of Veterans Affairs (to RJL), the Evenor Armington Fund (to RJL), and the OASI Institute for Research and Care (IRCCS) on Mental Retardation and Brain Aging, Troina, Italy (to AS). Jorge Otero-Millan is a Fellow of the Pedro Barrié de la Maza Foundation.